To modify processing, working and use characteristics, most plastics are provided with auxiliaries and with fillers and reinforcers. The latter improve properties such as stiffness, strength, heat resistance, dimensional stability, and reduce the thermal expansion of products based on plastics.
Of particular significance for plastics compositions are fillers and reinforcers composed of minerals or glass, especially borosilicate glass or silicate glass, which is used in a wide variety of different forms, for example in the form of glass fibres, glass flakes or else in the form of expanded glass or foamed glass. Fillers and reinforcers have a significant influence on the heat resistance of plastics. For example, when fibrous fillers having a correspondingly high aspect ratio are used, very good heat resistances are achieved. However, the anisotropic geometry of a fibre in the course of processing leads to alignment of the fibres in flow direction and to associated anisotropic shrinkage during processing, which subsequently leads to unwanted warpage in the products. The “wick effect” associated with the fibres also leads to a deterioration in the self-extinguishment properties of these products, these being of significance, for example, in the glow wire test to IEC 60695-2-12 (GWFI). In order to be able to assure sufficient flame retardancy of plastics-based products with fibrous fillers too, for example glass fibres, it is generally necessary to use halogen- or phosphorus-based flame retardants. Halogen-based flame retardants are the subject of public discussion because they accumulate in the environment. It is desirable to avoid phosphorus-based flame retardants because of energy-intensive production. Moreover, in the case of phosphorus-containing flame retardants, there is the risk of corrosive deposits at electrical contacts when the product is an electrical component or electronic component.
When non-fibrous fillers are used, especially talc, clay minerals, mica, expanded glass or foamed glass, isotropic shrinkage is obtained in products, but these moulding compositions and the products that are produced therefrom then frequently have inadequate heat resistances (<130° C.) or inadequate self-extinguishment properties in the GWFI test at relatively low wall thicknesses, especially wall thicknesses<1 mm.
EP 0361367 A2 discloses polyamide-based compositions comprising nylon-6, a hollow spherical filler and reinforcer made from glass and having a median particle size in the range from 5 to 200 μm (Glass Bubbles SSX) and chopped long fibres having a starting length of 3 mm (CSO3JAFT2), these having a high heat resistance.
EP 2468810 A1 Example 3 describes a polyamide-based composition comprising, as well as melamine cyanurate, ground glass and also ground chopped glass fibres. A disadvantage of this composition according to EP 2468810 A1 is its poor heat resistance and associated significantly restricted usability in electrical components, for example circuit breakers.
CN 103 013 104 A describes flame-retardant nylon-6-based compositions based on halogen-free flame retardants, comprising melamine cyanurate and talc as inorganic filler.
But a good heat resistance with simultaneously isotropic shrinkage characteristics and good self-extinguishment properties in the GWFI test is an important prerequisite for the use of polyamide-based compositions in electronic components of complex structure, especially in residual current circuit breakers and other circuit breakers.
According to “http://de.wikipedia.org/wiki/Leitungsschutzschalter”, a circuit breaker, also colloquially called cut-out or fuse for short, is an excess current protection device in electrical installation and is used in low-voltage grids. A residual current circuit breaker provides protection from fault currents (see: http://de.wikipedia.org/wiki/Fehlerstromschutzschalter).
It was therefore an object of the present invention to provide polyamide-based compositions suitable for production of products for the electrical industry, these products being notable for high heat resistance with simultaneously low isotropic shrinkage characteristics, and for good self-extinguishment properties in the glow wire test to IEC60695-2-12, even with low wall thicknesses, especially with wall thicknesses around 0.8 mm.
According to “http://de.wikipedia.org/wiki/W%C3%A4rmeformbest%C3%A4ndigkeit”, heat resistance is a measure of the thermal durability of plastics. Because they have viscoelastic material characteristics, there is no strictly defined upper use temperature for plastics; instead, a substitute parameter is determined under defined load. For this purpose, two standardized methods are available, the method of heat deflection temperature (HDT) and the Vicat softening temperature (VST).
The method of the heat deflection temperature described in DIN EN ISO 75-1, -2, -3 (precursor: DIN 53461) uses standard test specimens with rectangular cross section, which are subjected to three-point bending under constant load, preferably with their edges flat. According to the test specimen height, an edge fibre strain σ1 of 1.80 (Method A), 0.45 (Method B) or 8.00 N/mm2 (Method C) is achieved by using weights or/and springs to apply a force
  F  =                    2        ⁢                  σ          f                ⁢                  bh          2                            3        ⁢                                  ⁢        L              .                  b: sample width        h: sample height        L: distance between rests.        
Subsequently, the stressed samples are subjected to heating at a constant heating rate of 120 K/h (or 50 K/h). If the deflection of the sample reaches an edge fibre elongation of 0.2%, the corresponding temperature is the heat deflection temperature (or heat distortion temperature) HOT.
The Vicat softening temperature (VST) to DIN EN ISO 306 (precursor: DIN 53460) is measured with a needle (having a circular area of 1 mm2). A test force of 10 N (test force A) or 50 N (test force B) is applied thereto. The test specimen having a permissible thickness of 3 to 6.4 mm is subjected to a defined heating rate of 50 or 120 K/h. The VST has been attained when the penetrating body reaches a penetration depth of 1 mm. According to the standard, the test is only applicable to thermoplastics and gives an indication of the practical sustained use limit, which is about 15 K below the Vicat temperature. Variation of the boundary conditions gives four parameter combinations:                VST/A50        VST/A120        VST/B50 (preferred method for comparative tests (ISO 10350-1)        VST/B120.        
According to “http://de.wikipedia.org/wiki/Schwindung#Schwindung_bei_Gie.C3.9Fharzen”, shrinkage is the change in volume of a material or workpiece without removal of material or exertion of force. Shrinkage takes place through drying, cooling or chemical or physical transformation mechanisms in the material. Low shrinkage in casting resins based on thermoplastics is a quality criterion, since installed components can otherwise come under compressive stress, and gaps can form between these and other components to be wetted if adhesion is insufficient. In the case of injection-moulded products in electrical engineering/electronics, shrinkage can lead to ingress of moisture and to reduced stress resistance. Isotropic shrinkage is understood by the person skilled in the art to mean equal shrinkage in all spatial directions. The shrinkage characteristics are tested to DIN EN ISO 294-4, as is also the case in the context of the present invention.
It has now been found that, surprisingly, compositions based on PA 6 or PA 66, when glass beads are used in the form described in detail below in combination with glass fibres, melamine cyanurate and titanium dioxide, lead to electrical or electronic articles having excellent properties in relation to heat resistance, flame retardancy in the glow wire test to IEC60695-2-12 and isotropic shrinkage characteristics.